1. Field of the Invention
The invention relates to a field correlation detection apparatus of a digital image signal and a coding apparatus using such a detection apparatus.
2. Related Background Art
Hitherto, an orthogonal transformation coding system is known as a technique for high efficiently compressing an image signal.
According to the orthogonal transformation coding system, an image signal is divided into blocks each consisting of a predetermined number of pixels, and after that, an orthogonal transformation such as a discrete cosine transformation (DCT) or the like is executed, and a quantization, an entropy coding, or the like is executed to coefficients after completion of the transformation.
As such a DCT, there are two kinds of DCTs: a frame DCT in which a block of pixels of [m (vertical direction).times.n (horizontal direction)] (hereinafter, referred to as an image processing block) is converted in a lump as a frame image; and a field DCT in which a block of [m (vertical direction).times.n (horizontal direction)] is divided into two fields and transformed every field, namely, independently, and after that, an addition and a difference of coefficients are obtained (refer to FIG. 1: n=m=8).
It is known that when the field DCT is applied to a block of a large motion, namely, low interfield correlation, high frequency components in the vertical direction which occur due to the motion are concentrated to a low frequency side, so that a coding efficiency is generally better and a distortion due to a quantization and an inverse quantization is smaller than those in the case where the frame DCT is applied.
FIG. 2 is an explanatory diagram showing the sum of the absolute values of interfield differences of one column in the vertical direction in an image processing block.
When the frame DCT processing and the field DCT processing are selected, hitherto, the total (refer to FIG. 2) for each one DCT block of the sum of the absolute values of interfield differences of one column in the vertical direction is compared with a certain threshold value for every DCT block. The block in which the sum is equal to or less than the threshold value is decided to be a block of a small motion, so that the DCT is switched to the frame DCT. The block in which the sum is larger than the threshold value is determined to be a block of a large motion, so that the DCT is switched to the field DCT.
According to the conventional discriminating method, however, in the case where there are correlations every two lines in data in the DCT block, for instance, in the case where the data in the image processing block is as shown in FIG. 3A, a large difference occurs when the correlation of the data such that the data in the block is deviated by one line (FIG. 3B) is compared with the correlation of the data of FIG. 3A.
Namely, although the correlation is strong in FIG. 3A, since the correlation is weak in FIG. 3B, the field DCT processing is executed in FIG. 3B.
In this case, two field DCT blocks have a highest frequency. When the DCT block as shown in FIG. 3B is processed by a discriminating method like a conventional method, coefficients in a high frequency area are raised, so that there is a possibility such that a coding efficiency deteriorates.
A problem also occurs in the conventional discriminating method even in case of the following image.
FIGS. 4 and 5 are graphs showing DCT blocks such that the sums of the absolute values of the interfield differences of one column in the vertical direction are almost equal. For the DCT blocks shown in the diagram, according to the foregoing conventional discriminating method shown in FIG. 2, since the sums of the absolute values of the interfield differences of one column in the vertical direction are the same, the same DCT (field, frame) processing is executed by the threshold value for both of the DCT blocks.
FIG. 4 shows an image pattern in the case where there is a motion in an edge in the horizontal direction existing in a flat picture. FIG. 6 is a graph showing the result obtained by processing the block of the image pattern of FIG. 4 by the frame DCT. FIG. 7 is a graph showing the result obtained by processing the block of the image pattern of FIG. 4 by the field DCT. In the diagrams, it will be understood that as for the DCT coefficients, if they are processed by the field DCT, it is advantageous for a distortion by the quantization and inverse quantization.
On the other hand, FIG. 5 shows a high fine image of a relatively low saturation. FIG. 8 is a graph showing the result obtained by processing the block of the image pattern of FIG. 5 by the frame DCT. FIG. 9 is a graph showing the result obtained by processing the block of the image pattern of FIG. 5 by the field DCT. In case of the DCT block of the picture as shown in FIG. 5, since there is no tendency such that the DCT coefficients are concentrated to the low frequency side by the field DCT processing, there is a case where the number of significant coefficients is larger than that in the case where the block is processed by the frame DCT. The distortion due to the quantization and inverse quantization is equal to or larger than that in the case where the block is processed by the frame DCT.
As mentioned above, according to the conventional discriminating method shown in FIG. 2, the DCT blocks as shown in FIGS. 4 and 5 cannot be distributed to the proper DCT process.